1. Field of the Invention
This invention relates to a process for producing hollow microspheres and to the hollow microspheres so produced. More particularly, the invention relates to the production of hollow microspheres containing or consisting entirely of carbon, and to the production of hollow microspheres which are precursors of such carbon-containing hollow microspheres.
2. Discussion of the Prior Art
Microspheres made of carbon and other materials have numerous uses in industry. For example, they can be used for the preparation of metal foams and syntactic foams (hollow carbon spheres in a polymer matrix), for the formation of filter beds and for the production of lightweight carbon composites. One known method of producing carbon microspheres involves the carbonization of pellets made from pitch (Y. Amagi et. al. "Hollow Carbon Microspheres from Pitch Material and their Applications," SAMPLE 10th National Symposium 71), but pitch pellets can fuse together during carbonization unless steps are taken to avoid this by a time-consuming and expensive pretreatment.
Another method of forming hollow microspheres is disclosed in U.S. Pat. No. 2,797,201 to Veatch et. al. issued on June 25, 1957. This method involves forming droplets of a solution in a volatile solvent of a gas-generating material and a film-forming polyvinyl alcohol or phenolformaldehyde resin, and heating the droplets by a spray drying technique to form hollow microspheres of 1-500.mu. in size. However, this process does not result in particles of a uniform size and, indeed, is not effective at all for producing microspheres larger than about 0.5 mm (500.mu.) in diameter. This is because almost 70-85% of each droplet consists of solvent, so that, during the spray drying step, a large amount of heat must be transferred to the droplet in a short period of time in order to vaporize the solvent completely. This must take place while gas is being generated within the droplets and during the short time the droplets remain out of contact with each other, otherwise agglomeration will take place. All of this is extremely difficult in a spray drying system when the droplets exceed a certain maximum size.